Ultra-thin Ni—Fe-MOF nanosheet, preparation method and use thereof

Abstract

The present invention discloses a method for preparing an ultra-thin Ni—Fe-MOF nanosheet, which comprises the steps of dissolving an organic ligand in an organic solvent, dripping the resulting solution to an aqueous solution containing a nickel salt and an iron salt, mixing uniformly and reacting at 140-160° C. for 3-6 h to obtain the ultra-thin Ni—Fe-MOF nanosheet, wherein the organic ligand is terephthalic acid and/or disodium terephthalate, and the organic solvent is N,N-dimethylacetamide and/or N,N-dimethylformamide. The present invention discloses an ultra-thin Ni—Fe-MOF nanosheet, and use thereof. The preparation method does not require a surfactant, the surface of the product is neat and easy to be cleaned, and the large-scale synthesis of 2D ultra-thin MOF materials can be realized.

Claims

1. A method for preparing an Ni—Fe-MOF nanosheet, comprising steps of: dissolving an organic ligand in an organic solvent to get a mixed solution; dripping the mixed solution to an aqueous solution containing a nickel salt and an iron salt with stirring; and mixing uniformly and reacting at 140-160° C. for 3-6 hours, to obtain the Ni—Fe-MOF nanosheet; wherein the organic ligand is terephthalic acid and/or disodium terephthalate, and the organic solvent is N,N-dimethylacetamide and/or N,N-dimethyl formamide, wherein a volume ratio of water of the aqueous solution to the organic solvent is 1-2:1, and wherein the concentration of nickel ions in the reaction solution is 8.33 mmol/L, the concentration of iron ions is 2.50 mmol/L, and the concentration of terephthalic acid is 4.17 mmol/L.

2. The method for preparing an Ni—Fe-MOF nanosheet according to claim 1, wherein the nickel salt is nickel acetate tetrahydrate and/or nickel nitrate hexahydrate, and the iron salt is ferrous sulfate heptahydrate.

3. The method for preparing an Ni—Fe-MOF nanosheet according to claim 1, further comprises the steps of washing, centrifuging and drying the obtained product, wherein the solvent for washing is deionized water and ethanol, the rotation speed during the centrifugation is 8000-10000 rpm, the centrifugation time is 3-5 minutes, the drying temperature is 50-60° C., and the drying time is 10-12 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scanning electron microscope (SEM) image of an ultra-thin Ni—Fe-MOF nanosheet;

(2) FIG. 2 is a transmission electron microscope (TEM) image of an ultra-thin Ni—Fe-MOF nanosheet;

(3) FIG. 3 is a powder X-ray diffraction (PXRD) pattern of an ultra-thin Ni—Fe-MOF nanosheet;

(4) FIG. 4 is an infrared (IR) spectrum of an ultra-thin Ni—Fe-MOF nanosheet;

(5) FIG. 5 is an energy dispersive X-ray (EDX) spectrum of an ultra-thin Ni—Fe-MOF nanosheet;

(6) FIG. 6 is high-angle annular dark-field images of an ultra-thin Ni—Fe-MOF nanosheet showing corresponding element distribution;

(7) FIG. 7 is an atomic force microscope (AFM) image of an ultra-thin Ni—Fe-MOF nanosheet;

(8) FIG. 8 is an X-ray photoelectron spectrum (XPS) of an ultra-thin Ni—Fe-MOF nanosheet;

(9) FIG. 9 shows an SEM image (a), TEM images (b and c), a high-angle annular dark-field image showing corresponding element distribution (d) and an EDX spectrum (e) of a Ni-MOF nanosheet;

(10) FIG. 10 is a PXRD pattern of a Ni-MOF nanosheet;

(11) FIG. 11 is an IR spectrum of a Ni-MOF nanosheet;

(12) FIG. 12 are photographs showing the dispersion and powder of an ultra-thin Ni—Fe-MOF nanosheet prepared by the expanded reaction;

(13) FIG. 13 is a TEM image of an ultra-thin Ni—Fe-MOF nanosheet prepared by the expanded reaction;

(14) FIG. 14 is an XRD image of an ultra-thin Ni—Fe-MOF nanosheet prepared by the expanded reaction;

(15) FIG. 15 shows (a) the polarization curve and (b) the Tafel slope in an OER reaction catalyzed by an ultra-thin Ni—Fe-MOF nanosheet;

(16) FIG. 16 is a bar graph showing the overpotential and current density in an OER reaction catalyzed by an ultra-thin Ni—Fe-MOF nanosheet; and

(17) FIG. 17 shows the chronopotentiometric graph in an OER reaction catalyzed by an ultra-thin Ni—Fe-MOF nanosheet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(18) The present invention will be further described below with reference to the accompanying drawings and specific examples, so that those skilled in the art can better understand and implement the present invention; however, the present invention is not limited thereto.

Example 1: Preparation of Ultra-Thin Ni—Fe-MOF Nanosheet

(19) Solid nickel acetate tetrahydrate (24.8 mg, 0.1 mmol) and ferrous sulfate heptahydrate (7.38 mg, 0.03 mmol) were weighed and dissolved in deionized water (6 mL) to give a solution A. Terephthalic acid (8.3 mg) as an organic ligand was dissolved in DMAC (6 mL) to give a solution B. The solution B was slowly added to the solution A with stirring and mixed evenly. Then, the mixed solution was fed to a stainless steel reactor lined with polytetrafluoroethylene, sealed, and then reacted in an oven at 150° C. for 3 h. After the reaction, the reaction solution was naturally cooled to room temperature, washed with deionized water and ethanol, separated by centrifugation, and dried to obtain an ultra-thin Ni—Fe-MOF nanosheet as a yellow powder, which was designated as Ni—Fe-MOF NSs.

(20) As shown in FIGS. 1 and 2, the ultra-thin Ni—Fe-MOF nanosheet has a uniform morphology, high quality and yield.

(21) As shown in FIG. 3, the powder X-ray diffraction pattern (PXRD) of the ultra-thin Ni—Fe-MOF nanosheet is consistent with that of the simulated MOF material (CCDC No. 638866 and JCPDS No. 00-035-1677).

(22) As shown in FIG. 4, the IR spectrum of the ultra-thin Ni—Fe-MOF nanosheet also confirms the presence of hydroxyl, terephthalate and sulfate in the synthesized ultra-thin Ni—Fe-MOF nanosheet.

(23) As shown in FIGS. 5 and 6, the ultra-thin Ni—Fe-MOF nanosheet is composed of Ni, Fe, C, O and S elements, where each element is evenly distributed.

(24) As shown in FIG. 7, the thickness of the ultra-thin Ni—Fe-MOF nanosheet is between 1.67 nm and 2.58 nm.

(25) As shown in FIG. 8, the X-ray photoelectron spectrum (XPS) of the ultra-thin Ni—Fe-MOF nanosheet shows that the valences of Ni, Fe and S are +2, +3 and +2, respectively.

Comparative Example 1: Preparation of Single Metal Ni-MOF Nanosheet

(26) Single metal Ni-MOF nanosheet has a chemical formula of [Ni.sub.3(OH).sub.2(1,4-BDC).sub.2(H.sub.2O).sub.4].2H.sub.2O, where Ni represents nickel ion, OH represents bridged hydroxyl ligand, 1,4-BDC represents deprotonated terephthalic acid ligand, the preceding H.sub.2O represents water molecules involved in coordination, and the latter H.sub.2O represents free water molecules. The preparation process was as follows.

(27) Solid nickel acetate tetrahydrate (24.8 mg, 0.1 mmol) was weighed and dissolved in deionized water (6 mL) to give a solution A. Terephthalic acid (8.3 mg) as an organic ligand was dissolved in DMAC (6 mL) to give a solution B. The solution B was slowly added to the solution A with stirring and mixed evenly. Then, the mixed solution was fed to a stainless steel reactor lined with polytetrafluoroethylene, sealed, and then reacted in an oven at 150° C. for 3 h. After the reaction, the reaction solution was naturally cooled to room temperature, washed with deionized water and ethanol, separated by centrifugation, and dried to obtain a single-metal Ni-MOF nanosheet as a light-blue powder.

(28) As shown in FIG. 9, the Ni-MOF material has a two-dimensional flake morphology, and is mainly composed of Ni, C and O elements, where each element is uniformly distributed.

(29) As shown in FIG. 10, the powder X-ray diffraction pattern (PXRD) of the Ni-MOF nanosheet is consistent with that of the simulated MOF material (CCDC No. 638866 and JCPDS No. 00-035-1677).

(30) As shown in FIG. 11, the IR spectrum of the Ni-MOF nanosheet also confirms the presence of hydroxyl and terephthalate.

Example 2: Mass Preparation of Ultra-Thin Ni—Fe-MOF Nanosheet

(31) Solid nickel acetate tetrahydrate (2.48 g, 10 mmol) and ferrous sulfate heptahydrate (0.738 g, 3 mmol) were weighed and dissolved in deionized water (300 mL) to give a solution A. Terephthalic acid (0.830 g, 5 mmol) as an organic ligand was dissolved in DMAC (300 mL) to give a solution B. The solution B was slowly added to the solution A with stirring and mixed evenly. Then, the mixed solution was fed to a 1000 mL thick-wall reaction flask, sealed, and then reacted in an oven at 150° C. for 3 h. After the reaction, the reaction solution was naturally cooled to room temperature, washed with deionized water and ethanol, separated by centrifugation, and dried to obtain an ultra-thin Ni—Fe-MOF nanosheet as a yellow powder (1.7486 g), as shown in FIG. 12.

(32) As shown in FIG. 13, the mass-synthesized ultra-thin Ni—Fe-MOF nanosheet has a uniform morphology, high quality and yield.

(33) As shown in FIG. 14, the powder X-ray diffraction pattern (PXRD) of the mass-synthesized ultra-thin Ni—Fe-MOF nanosheet is consistent with that of the simulated MOF material (CCDC No. 638866 and JCPDS No. 00-035-1677).

Example 3: Preparation of Electrocatalyst

(34) Ultra-thin Ni—Fe-MOF nanosheet solid powder (2.5 mg) and commercial carbon black (2.5 mg) were weighed and mixed. Isopropanol (970 μL) and 5 wt. % Nafion solution (30 μL) were added, and ultrasonicated for 1 h to disperse the solid evenly to form an ink-like solution. The solution (20 μL) was dripped onto a polished surface of a glassy carbon electrode in batches, and let it dry naturally before use.

(35) As a comparison, Ni-MOF solid powder (2.5 mg) and commercial carbon black (2.5 mg) were weighed and mixed. Isopropanol (970 μL) and 5 wt. % Nafion solution (30 μL) were added, and ultrasonicated for 1 h to disperse the solid evenly to form an ink-like solution. The solution (20 μL) was dripped onto a polished surface of a glassy carbon electrode in batches, and let it dry naturally before use.

(36) As a comparison, commercial Ir/C (5 wt. % Ir) (5.0 mg) was weighed. Isopropanol (970 μL) and 5 wt % Nafion solution (30 μL) were added, and ultrasonicated for 1 h to disperse the solid evenly to form an ink-like solution. The solution (20 μL) was dripped onto a polished surface of a glassy carbon electrode in batches, and let it dry naturally before use.

Example 4: OER Performance Test

(37) The entire electrocatalytic test was carried out with a standard three-electrode system, where the working electrode was a glassy carbon electrode coated with the catalyst, the reference electrode was an Ag/AgCl (saturated KCl solution) electrode, and the auxiliary electrode was a platinum wire electrode. The electrolyte solution used for linear sweep voltammetry (LSV) test was 1 M KOH solution saturated with O.sub.2, the sweep range of the potential was 0-0.8 V, the sweep speed was 5 mV/s, and the test data were all compensated by iR.

(38) As shown in FIGS. 15 and 16, compared to the single metal Ni-MOF nanosheet and the commercial Ir/C, the ultra-thin Ni—Fe-MOF nanosheet exhibits excellent electrocatalytic performance in the OER reaction, the overpotential value is only 221 mV at a current density of 10 mA.Math.cm.sup.−2, and the Tafel slope is as low as 56.0 mV.Math.dec.sup.−1.

Example 5: Electrocatalytic Stability Test

(39) In a standard three-electrode system, the reference electrode, the auxiliary electrode, and the glassy carbon electrode coated with the catalyst were inserted into a 1 M KOH solution saturated with O.sub.2 to perform a chronopotentiometric test. The current value of the test was 2 mA.

(40) As shown in FIG. 17, the ultra-thin Ni—Fe-MOF nanosheet shows excellent stability. In the constant-current chronopotentiometric test, the electrocatalytic performance has no significant decrease after 20 h.

(41) The above-described embodiments are merely preferred embodiments for the purpose of fully illustrating the present invention, and the scope of the present invention is not limited thereto. Equivalent substitutions or modifications can be made by those skilled in the art based on the present invention, which are within the scope of the present invention. The scope of the present invention is defined by the appended claims.